1. Field of the Invention
The invention relates to thermal treatment of metals in a controlled atmosphere.
2. Discussion of the Background
A controlled atmosphere is essential for the annealing of metallic pieces. It is conventionally done in the following manner:
either by using an exothermic generator providing incomplete combustion of a hydrocarbon and air that yields combustion gases which, possibly after purification, contain hydrogen and carbon monoxide. Both of these gases are reducers at their respective concentrations which depend on the ratio of air/hydrocarbon admitted into the generator. As an example, such an exothermic atmosphere can contain 5 to 10% carbon monoxide and 6 to 12% hydrogen;
or by creating a synthetic atmosphere based on pure industrial gases such as nitrogen and hydrogen. Nitrogen is produced by cryogenic distillation of air and contains very few impurities. For example, the total of water vapor and oxygen impurities is generally less than 10 ppm by volume. Hydrogen, or a hydrocarbon, or hydrogen and a hydrocarbon, or methanol is added to this very pure nitrogen in such a manner as to produce a reductive atmosphere and, if necessary, to produce a non-decarbonizing atmosphere, which is used to treat the metallic pieces.
This second method of operation has the advantage of completely controlling the quality of the treatment atmosphere but has the disadvantage of using cryogenic nitrogen, which is relatively expensive. This is the reason why an effort is usually made to reduce the flow of gas admitted to the system by creating a nitrogen cushion, particularly at the exit from the cooling zone, in order to prevent any back-flow of air through the cooling zone. This procedure results in a significant reduction of the overall flow admitted. However, in spite of this major reduction in flow, industrially pure gases are still far from being economically attractive relative to gases produced by an exothermic generator.
This is the reason why, in certain applications where this has proven to be possible, it has been proposed to replace the cryogenic nitrogen with nitrogen produced by separation of air according to adsorption or selective permeation techniques. Under certain production conditions, these techniques are less costly than cryogenic nitrogen. However this is at the detriment of oxygen impurity, since nitrogen produced by adsorption usually contains a residual content of oxygen of 0.5% to 5% while the residual content of oxygen in nitrogen produced by permeation generally exceeds 3% and can go as high as 10%.
This oxygen impurity makes it very difficult to use the raw nitrogen directly for producing a suitable thermal treatment atmosphere. In practice, it has been proposed that nitrogen produced according to the selective permeation process be used only for production of atmospheres produced from nitrogen and methanol, as is described in the article "Heat Treating Processes With Nitrogen and Methanol Based Atmosphere" M. Kostelitz et at. in Journal of Heat Treating, Volume 2, No. 1-35, and in the French Patents 79.05.599, 82.12.380 and 85.12.379, in the name of the applicant. Such an atmosphere formed on the basis of nitrogen with a residual content of oxygen and containing methanol can, theoretically, be used in different applications, namely heating before quenching, carbonitriding, and steel cementation. But it is only in the latter area that the use of nitrogen with a residual content of oxygen has been used industrially, and this is due to the fact that the elevated temperature which cementation implies, on the order of 900.degree. C., promotes reaction of the residual oxygen carried by the nitrogen with chemicals of the hydrocarbon type which are simultaneously admitted to form the base atmosphere.
It has been proposed to purify the nitrogen produced by adsorption or permeation, having a residual content of oxygen, by having the oxygen react catalytically with a corresponding supply of hydrogen sufficient to assure complete elimination of all the oxygen. But this relatively expensive process results in a production cost almost equal to that of cryogenic nitrogen, which speaks against this form of production of pure nitrogen, especially since production of nitrogen by adsorption or permeation does not have the advantages of flexibility and simplicity that production of cryogenic nitrogen has.